Recent development and applications of poly (ionic liquid)s in microextraction techniques

Highlights

• Recent development and applications of PIL-based sorbents in microextraction were reviewed.

• The current problems and future prospects of PIL-based sorbents were overviewed.

• Microextraction formats such as SPME, MMF-SPME, IT-SPME, MSPE, SCSE, SBSE and TBME were discussed.

Abstract

Microextraction techniques have been developed rapidly in recent years due to their numerous advantages including simplicity, miniaturization, cost-effectiveness, easy automation, low consumption of sample and eco-friendliness. Poly (ionic liquid)s (PILs) belong to a subclass of polyelectrolytes which are characterized by a series of unique physicochemical features including high thermal, chemical and mechanical stability, excellent plasticity and spatial controllability. PILs have attracted considerable attention in sample pretreatment. This review mainly concentrates on the recent development and applications of PIL-based sorbents in microextraction techniques. The properties of PIL-based sorbents, their general preparation approaches, application in enrichment, the current problems and future prospects are overviewed. The formats of microextraction techniques discussed in this review include solid phase microextraction, monolithic fibers solid phase microextraction, in-tube SPME, magnetic solid phase extraction, stir bar sorptive extraction, stir cake sorptive extraction and tip-based microextraction.

Introduction

The development and applications of highly sensitive and precise analytical instruments (e.g., HPLC or GC with MS/MS detector) provide solid support for the accurate quantification of trace analytes in complex samples. However, in most cases, direct analysis of compounds is rather difficult due to the much complex compositions, high levels of matrix interference and the low concentrations of target analytes in real samples. Therefore, proper sample preparation must be employed prior to instrumental analysis. Among the numerous sample pretreatment methods, liquid-liquid extraction (LLE) and solid phase extraction (SPE) are considered as the classic methods due to their salient extraction performance and high recoveries. They have been widely applied for the determination of multifarious compounds in various samples [1], [2]. Nevertheless, it must be noted that, despite of their universal applications, LLE and SPE are accompanied with drawbacks of tedious and time-consuming owing to the multi-step procedures. Apart from that, large volume of toxic organic reagents is required, producing a huge amount of waste eventually.

In recent years, the concept of “Green Analytical Chemistry (GAC)” involving in the sustainable development and searching for new eco-friendly analytical techniques has become a hot topic. Many efforts have been made by chemists in response to the requirements of GAC. In this trend, many microextraction techniques based on liquid sorbent like dispersive liquid-liquid microextraction (DLLME) [3], single drop microextraction (SDME) [4], or solid sorbent such as solid phase microextraction (SPME) [5], in-tube SPME (IT-SPME) [6], stir bar sorptive extraction (SBSE) [7] and chip-based microextraction (CBME) [8] have sprung up. Different types of microextraction techniques are presented in Fig. 1. These techniques downsize the extraction device, minimize the consumptions of sample and organic solvent, and simplify the pretreatment procedures. Furthermore, part of them, such as IT-SPME and CBME, are easy to be online combined with analytical instrumentations to achieve fully automated and high-throughput analysis, which is beneficial for the reduction of analytical time as well as the utmost minimization of sample losses and contaminations. For all microextraction techniques, the extraction performance for analytes depends on the types of sorbents or extraction phase. Therefore, development and introduction of novel sorbents with high sorption capacity, good extraction efficiency and selectivity are important for the advance of microextraction techniques. At present, various micro/nanomaterials have been employed as sorbents for microextraction. Some examples are silica [9], carbon (carbon nanotubes, graphene) [10], metal-organic frameworks (MOFs) [11] and monoliths [12].

Besides the aforementioned materials, one kind of “green” reagent named ionic liquids (ILs) has attracted considerable attention. ILs refer to a group of molten salts that keep in liquid at temperatures below 100°C or at room temperature in most cases. Generally, ILs consisted of large organic cations (e.g., imidazolium, pyridinium, pyrrolidinium or quaternary ammonium) and small inorganic or organic anions (e.g., trifluoromethylsulfonate, trifluoroethanoate or Cl⁻, Br⁻, PF₆⁻) [13]. The fascinating thing is that the different combination of cations and anions can constitute ILs with different physicochemical properties, so ILs are also called “designer solvents” and their structures can be tuned and modulated according to the specific demands and applications. Furthermore, ILs are considered as ideal extraction media because they can provide multi-interactions with target molecules, including hydrophobic/hydrophilic interactions, hydrogen bonding, π-π, ion-exchange and electrostatic interactions. At the same time, the structural flexibility enables the incorporation of different functional groups within ILs according to the properties of analytes. Based on above-mentioned merits, there has been a growing interest in the integration of ILs with some microextraction techniques such as SPME [14], DLLME [15] and SDME [16]. The details can be found in some well-documented reviews [17], [18], [19]. However, there are still some problems during their applications. The main problems faced by ILs are their instability, vulnerability and low utilization efficiency [17]. For instance, Hsieh et al. [20] prepared a Nafion membrane supported IL fiber coating for SPME. The fiber was applied to enrich polycyclic aromatic hydrocarbons (PAHs) from water samples. Although the presence of Nafion membrane enhanced the quantity and stability of IL film, the fiber had to be re-coated with IL after each extraction due to the loss of the IL in desorption procedure at high temperature, which significantly reduced the convenient nature of SPME. The same dilemma was encountered in Liu's study [21]. In order to overcome above-mentioned disadvantages without sacrificing the advantages of ILs, poly (ionic liquid)s (PILs) are introduced into the microextraction techniques. PILs are a group of polyelectrolytes derived from IL monomers. They incorporate the unique properties of ILs into polymer chains. Accordingly, PILs show higher thermal stability, improved durability, better mechanical robustness and longer lifetime than ordinary ILs. More than that, PILs also exhibit the same characteristics including processability, plasticity and spatial controllability as ordinary polymers [22]. As promising candidates for sorbents, PILs display wide applications in microextraction.

In order to provide comprehensive information of PILs and their applications in microextraction, the first part of this review discusses the basics of PILs in terms of their classification, synthesis and properties for microextraction. In the second part, the review emphasizes the recent development and applications of PILs as sorbents in mircoextraction technologies, mainly focusing on SPME, multiple monolithic fibers solid phase microextraction (MMF-SPME), IT-SPME, magnetic solid phase extraction (MSPE), SBSE, stir cake sorptive extraction (SCSE), tip-based microextraction (TBME). The current drawbacks and development prospects are also been discussed.